The administration of inhaled nitric oxide (NO) to patients is currently being investigated for its therapeutic effect. The use of NO has a vasodilatory effect on such patients and is particularly of importance in the case of newborns having persistent pulmonary hypertension. In such cases, the administration of NO has significantly increased the oxygen saturation in such infants.
The function of the administration of NO has been fairly widely published and typical articles appeared in The Lancet, Vol. 340, October 1992 at pages 818-820 entitled "Inhaled Nitric Oxide in Persistent Pulmonary Hypertension of the Newborn" and "Low-dose Inhalational Nitric Oxide in Persistent Pulmonary Hypertension of the Newborn" and in Anesthesiology, Vol. 78, pgs. 413-416 (1993), entitled "Inhaled NO--the past, the present and the future".
The actual administration of NO is generally carried out by its introduction into the patient as a gas along with other normal inhalation gases given to breathe the patient. Such commercially available supplies are provided in cylinders under pressure and may be at pressures of about 2000 psi and consist of a mixture of NO in nitrogen with a concentration of NO of between about 800-2000 ppm. As such, therefore, some means must be used to reduce the pressure of the supply to acceptable levels for a patient and also to very precisely meter the amount of the NO and nitrogen mixture so that the desired concentration of NO is actually administered to the patient. Such administration must also be added in sympathy with the respiration pattern of the patient.
The concentration administered to a patient will vary according to the patient and the need for the therapy but will generally include concentrations at or lower than 150 ppm. There is, of course, a need for that concentration to be precisely metered to the patient since an excess of NO can be harmful to the patient. In addition, the administration must be efficient in a timely manner in that NO is oxidized in the presence of oxygen to nitrogen dioxide and which is a toxic compound. Therefore, care in its administration is paramount.
Current known methods of such administration, therefore have been limited somewhat to clinical situations where attending personnel are qualified from a technical sense to control the mixing and administration of the NO to a patient. Such methods have included the use of a forced ventilation device, such as a mechanical ventilator where a varying flow os breathing gas is delivered to the patient as well as gas blenders or proportioners that supply a continuous flow of the breathing gas to the patient to which NO has been added.
In the former case, the use of a ventilator is constrained in that the user must know the precise flow from the ventilator and then the amount of NO to be added is determined on a case-to-case and moment-to-moment basis. Furthermore, the flow profile in forced ventilation varies continuously thereby making it impossible to track the flow manually. In the use of the latter gas blenders, the introduction of the NO containing nitrogen has been accomplished through the use of hand adjustment of the gas proportioner in accordance with a monitor that reads the concentration of NO being administered to the patient. Thus the actual concentration is continuously being adjusted by the user in accordance with the ongoing conditions of the apparatus providing the breathing mixture.
While such modes of providing a known concentration of NO to the patient may be acceptable from a closely controlled and monitored clinical setting, it is advantageous to have a system that could be used with various means of providing the breathing gas, whether by mechanical means such as a ventilator, or by the use of a gas proportioner and which could automatically adjust for that particular equipment and assure the user that the desired, proper concentration of NO is being administered to the patient.